1. Field of the Invention
The present invention relates to a laser recording apparatus which is so designed that the spot radius of a laser beam can be varied with a simple construction.
2. Description of the Prior Art
Recently, a laser recording apparatus (a so-called laser printer) which records an image by using a laser beam has been developed and put to practical use. The laser recording apparatus is so devised that a laser beam is modulated by an electric signal signifying an image information, and this image information is formed on a photosensitive member by the laser beam and recorded thereon as a visual image by an electrophotographic process or the like.
In the case when recording magnification is varied in the laser recording apparatus of this kind, it is desirable that the spot radius of a laser beam on a recording medium is made variable in accordance with said magnification.
FIG. 4 shows schematically the construction of a scanning optical system of a prior-art laser recording apparatus. Numeral 1 denotes a laser light source such as a semiconductor laser, 2 a collimator lens for making a beam parallel and forming it in a desired shape, 3 a beam expander for expanding the radius of the beam, 4 a rotary polyhedral mirror for scanning which rotates at high speed for deflecting the beam, 5 an f.theta. lens forming an image at a distance proportional to an incident angle .theta. to an optical axis, and 6 a photosensitive member, a recording medium, on which an electrostatic latent image of an image is recorded.
With a view to varying the spot radius of a laser beam, a method has been proposed heretofore (in Japanese Patent Laid-Open No. 17716/1985, for instance) in which the beam expander 3 is constructed of two sets of lens systems 3a, 3b, and 3c, 3d, for instance, and these sets of the lens systems are switched over for employment in accordance with the recording magnification at the time of selection by a rotating operation, as shown in FIG. 4. A method in which a zoom lens is used in place of the beam expander 3 is known as well.
Both of these methods have problems that they involve an increase in the sizes of a switching mechanism and the apparatus itself, that the radius of a beam spot can not be varied consecutively in the method in which the switchable beam expander is employed, and that a zoom lens mechanism is very expensive in the method in which the zoom lens is employed.
By the way, the optical intensity of a laser beam is in a Gaussian distribution as shown in FIG. 5, and the distribution of said intensity is expressed by the following equation. EQU I=I.sub.O e.sup.-a.spsp.2.sup.x.spsp.2
(where a is a constant). The radius of a beam at a given time can be determined from the value of x at the time when the optical intensity I is 1/e.sup.2 of the maximum intensity I.sub.O, and therefore a beam radius D.sub.O in this case is EQU D.sub.O =2.times.1/a.
This value is constant irrespective of the maximum optical intensity I.sub.O.
However, as is apparent from FIG. 6, in which the threshold value of an optical intensity required for a laser beam to be recorded on a recording medium is denoted by .alpha., the raduis d of a spot recorded actually varies in accordance with the difference in the optical intensity even when the radius of the beam is constant. That is, the spot radius d increases (d.sub.1 &gt;d.sub.0) with an increase in the optical intensity I (I.sub.1 &gt;I.sub.0).